THEEFFECTOF POLARITYREVERSALS ONTHEDIELECTRIC STRENGTH OF OIL-IMPREGNATED PAPERINSULATION FORH.V.d.c.CABLES F.K. Padgham P.Metra

A laboratoryinvestigationinvolvingtests on models and on full-sizedcables hasbeencarriedout intothebehaviourof oil-impregnated paper insulation underthesimultaneousaction of a pre-stressing direct voltageand a voltagesurge of eitherthesame or oppositepolarity. It has beenfoundthat the dielectricstrengthdoes not dependon thelevelor polarityof the pre-stressingdirectvoltageor on the durationof the surge, exceptwhena seriesof reversedpolaritysurges is appliedat intervalsof a fewseconds,progressively increasingtheleveluntil breakdownoccurs. In thistypicalcase reductions from130MV/mat zero d.c.to 45MV/mat 100 MV/md.c. have been observedandexplainedin termsofbubble formationandgrowth in the paper tape'butt-spaces" adjacentto themetallicconductors. Thiseffect is entirelyeliminatedby increasingtheoilpressure to suppress bubbleformation. From a practicalpoint of view theworkshows thatH.V.d.c.oil-filledcablesshouldnotbe regarded as being weak underpolarityreversalsunlessthese occur in rapid succession. If sucha situationis envisagedthe cable system can stillbe madecompletelyreliablefromthispointof viewby increasing the staticoil pressure to a suitablelevel.